1. Field of the Invention
The present invention relates to the field of recorded digital media. More specifically, the present invention pertains to a system and method for encoding, embedding, retrieving and decoding copy protection information within digital data.
2. Background Art
The use of digital media is proliferating due to the rapid development and increasing popularity of digital equipment such as digital videocassette recorders (VCRs), digital camcorders, digital still cameras and the like. The proliferation of digital media has also created a need for methods for enforcing copyrights and protecting copyright ownership of digital still images (including text) and video. For instance, the Copyright Protection Technique Group (CPTG) was formed as an industry consortium and is addressing copyright protection for digital video (or versatile) devices (DVDs).
One prior art method for protecting copyrights is to encrypt the copyrighted digital data. Only a valid keyholder is provided with a key for deciphering the encrypted data. However, once such data are decrypted, cryptographic techniques provide no method for controlling subsequent reproduction or retransmission. Thus, these techniques provide no protection against unauthorized reproduction of digital information.
To address the shortcomings associated with cryptographic techniques, copy protection information (e.g., a digital watermark) can be used to permanently embed an identification code into the digital data. The existence of visual redundancy in still images and video makes it possible to alter the digital image and video data in order to embed watermark information. The embedded watermark information serves as,the proof of copyright ownership. The watermark is intended to remain present within the data even after a decryption process is applied.
The watermark may be either invisible or visible; in the former case, it is desirable that the watermark be undetectable by the human visual system. It is also desirable that the watermark be robust; that is, the watermark should be difficult if not virtually impossible to remove or destroy, and attempts to remove or alter the watermark should also reduce the fidelity of the copyrighted data. In addition, it is desirable that the watermark unambiguously identify the proper owner of the copyrighted information.
In some prior art watermarking schemes, a pseudo-random sequence representing the watermark is inserted into the digital data; however, these schemes introduce a number of difficulties. First, the sequence must be large enough so that it will be unique, but this causes problems with detecting the watermark. For example, the detection metrics have to be computed for each of the watermark sequences issued for use in order to determine a statistically satisfactory match between any sequence detected in the data and a valid watermark. However, for a reasonably large sequence, this computation can take a very long time to complete.
Second, in order for the watermark to be robust against attempts to remove or destroy it by the introduction of noise, it must be different enough from other valid watermarks. Accordingly, the distance between each new watermark data and previously issued watermark data must be checked to make sure that the watermarks are not too close to each other. If they are too close, a new watermark must be generated and tested again. Again, these computations can take a very long time to complete. The random sequence techniques are also problematic if the watermark is limited in size in order to keep computation times to a reasonable length, because the robustness of the watermark is thereby reduced.
Another common prior art watermarking technique is to repeat the same watermark sequence at several different locations in the digital data. However, the resulting watermark is not robust against noise introduced to remove or destroy the watermark. For example, the watermark may be removed from some locations and remain in others, thus creating a degree of uncertainty regarding whether or not the watermark information was deliberately tampered with.
Another significant disadvantage to the watermark techniques described above and in other prior art watermark techniques is that knowledge of the original digital data (e.g., the original image) and of the original watermark is required in order to allow a comparison with the data in hand so that the watermark can be detected and decoded. Thus, for example, it is necessary to provide the digital equipment (e.g., VCRs, camcorders, and cameras) with the capability to store both the original set of digital data as well as the copy of the data in question and to compare the two sets of data; otherwise, additional equipment is necessary. In either case, the complexity of the equipment required and the associated cost are increased.
In addition, the prior art is problematic if digital data for either the original image or the copy is not available; for example, if only a print or film image is available. In this case, it is necessary to first scan the image for which the data are missing in order to have a digital representation of both the original and the copy so that a comparison can be performed to verify copyright authenticity. In addition to being a cumbersome approach, the scanning process requires additional equipment (and hence further increases costs), and also introduces uncertainties into the data that may in fact preclude detection of a valid watermark. Thus, in order to create a watermark that will remain detectable despite these uncertainties, it is necessary to significantly alter the digital data. For example, the pixel value of the data at each of the particular locations (e.g., a pixel) where the watermark is being embedded needs to be increased or decreased by a significant percentage so that the watermark will stand out even with the uncertainties introduced by scanning. Consequently, surrounding pixels may also need to be changed so that the image is properly blended. This in turn limits the number and position of possible locations within the digital data which can be altered without being detectable by the human visual system or without compromising the accuracy of the stored digital image.
In summary, prior art techniques for embedding watermarks into digital data representing still images and video suffer from a number of disadvantages. The watermarks generated using the prior art techniques are not robust against noise and are difficult to encode, detect and decode. The prior art techniques require knowledge of the digital data for the original image, and require complex and costly equipment for comparing the copy at hand with the original (and for scanning images when either the original or the copy are not available in digital form). The watermarks generated using prior art techniques can require significant alteration of the digital image data, and so the number and position of possible locations within the digital data for placing the watermark are limited.